electrical resistivity and vlf-em methods for the study of...

Nigerian Journal of Science Vol 49 (2015): 23-36

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Introduction

Heavy metals are natural components of Earth's crust that can neither be degraded nor destroyed and their contamination of soil is one of the most dangerous environmental problems throughout the world because of their ability to accumulate and cause toxicity in biological systems of humans, animals, microorganisms and plants (Nwachukwu et al., 2010). They enter the human body through food, water and air. Heavy metals are ubiquitous; therefore they tend to bioaccumulate thus causing an increase in their

concentration in biological systems. Depending on the size, quality and composition of the waste dumped by people, alteration in composition of the soil becomes inevitable. Heavy metal contamination of the soil environment has been occurring for centuries but its extent has increased remarkably in the last fifty years (Guanlin et al., 2005) due to technological development and increased consumer use of material containing these metals. Mining, manufacturing, and the use of synthetic products (e.g. pesticides, paints, batteries, industrial waste, and land application of industrial or domestic sludge) can result in heavy metal contamination of urban and agricultural soils. Potentially contaminated soils may occur at old landfill sites (particularly those that accept

Abstract

This study highlights the application of vertical electrical sounding (VES) and Very Low Frequency Electromagnetic (VLF-EM) methods as mapping tools for detection of subsurface conductive layers and depth of heavy metal contamination at Lalupon Lead dumpsite located at the outskirt of Ibadan, Oyo State, Southwestern Nigeria. In all, fourteen VLF-EM traverse lines were established with a portable ABEM WADI meter and fourteen VES were conducted using a resistivity meter around the perimeter of the dump site with maximum distance of 150m at an interval of 15m from the dump site outwardly. Water taken from hand dug well around the dumpsite were analyzed to determine the concentration of heavy metals (Pb, Zn, Cr, Cu, and Mn). Computer iteration of the sounding data was obtained using WINRESIST 1.0 version on data from partial curve matching on bi-logarithmic graph. Results from Southern region showed increase in resistivity in the first layer and relatively low resistivity in the second layer ranging from 6.1 ? m – 53.4

? m and thickness 2.6 m – 7.8 m which correlate with the conductive subsurface structural zones as shown by VLF-EM results. The control experiment indicated an aquiferous area while the water analysis suggests dominance of Pb (2.92 mg/l) and Mn (0.55mg/l) when compared with WHO and SON standard in well Y; therefore, results of the investigation indicate that the contaminant was spreading in the Southern region of the dump site.

Electrical Resistivity and VLF-EM Methods for the Study of Spread of Some Heavy Metal Contaminants in an Old

Lead-Battery Dumpsite

1 2*J.A. ADEGOKE AND E.O. OLUFOSOKAN

1Department of Physics, University of Ibadan

2Department of Physics, Lead City University, Ibadan.

*Corresponding AuthorE-mail: [email protected]

Keywords: Heavy metals, Contamination, Subsurface conductive layers, VLF-EM.

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industrial wastes), old orchards that used insecticides containing arsenic as an active ingredient, fields that had past applications of waste water or municipal sludge, areas in or around mining waste piles and tailings, industrial areas where chemicals may have been dumped on the ground, or in areas downwind from industrial sites(Popoola et al, 2011). The transportation of these elements into the groundwater flow systems could have serious impact on the receiving environment. Therefore, in order to develop an appropriate environmental water management plan in the polluted areas, it is a crucial task to detect the subsurface extension of pollution plume in vicinity of such dumps (Popoola et al, 2011).

There are many techniques to investigate the contamination zones associated with waste dumps. Among them, the geophysical methods have been used as a cost-effective approach. Since the various metals produced by oxidation processes in the groundwater flow system may change considerably the conductivity of the contaminated zone, the electric and electromagnetic (EM) geophysical methods could effectively be used to map these zones (Reynolds 1997). The use of geo-electrical and electromagnetic methods applied to landfills studies are well documented (Porsani et al., 2004; Karlik et al., 2001; Benson et al, 1997; Mukhtar et al, 2000; Fatta et al, 2000). For resistivity surveys, we need to install many

Figure 1: Map showing Lalupon and the surrounding villages, the study area (Ogundiran and Osibanjo, 2008)

J.A. Adegoke & E.O. Olufosokan: Electrical Resistivity & VLF-EM Methods for the Study of Spread of Some Heavy Metal Contaminants ...


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electrodes into the ground to send electrical current and record the potential differences between the specified electrodes to measure the subsurface resistivity changes of an intended area within the earth while for the VLF-EM method, only the components of the elliptically polarized magnetic field are measured using radio signals in the bandwidths of 15- 30 kHz for quick detection of near surface structures. In this research, the extension and dispersion of heavy metal contamination that originated from a lead-battery dump site was investigated using VLF-EM to highlight possible structural features and electrical resistivity sounding (VES) to delineate the subsurface features.

Materials and Methods

The study was carried out at Lalupon, a town located in Lagelu local government area of Oyo State within which was an old lead-battery dump site located between latitude 7°33'N - 7°34'N and longitude 4°06'E - 4°07'E and is surrounded by a rock boundary (Fig.1). The climatic condition of the area is tropical with two major seasons; rainy season from March to October and dry season from November to February. The vegetation of the area is affected by the alternation of these two seasons. The study area is part of the basement complex of Southwestern Nigeria. The map of Lalupon and the surrounding villages is presented in Fig 1. The dominant rock types in

Figure 2: Field layout of the geophysical survey.

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Ibadan region are quartzite, banded gneiss and granite gneiss of the metasedimentary series, banded gneiss, augen gneiss and migmatite constituting the gneiss-migmatite complex. Quarts schist outcrops occur as long ridges with relatively high elevation, which made them to be seen conspicuously. Their strike lines run in the north-south direction, between 40° and 350° with a consistent easterly dipping (Olayinka and Olayiwola, 2001). Banded gneiss outcrops in the Western and North-eastern part of Ibadan. They strike along the North direction with average dip angles of 47°W and 36°E. They are obliterated in some places by intrusive veins and dykes. Minor structures such as folds, shear zones, pinch and swell structures, concordant and discordant quartz veins and quartzo-feldspathic intrusions are present on the banded gneiss. Granite gneiss covers small portion of Ibadan. They rarely outcrop and where they do, boulders appear with it. The general strike direction trends in N-S with dip angle of 47°E and characterized by joints and faults in some cases (Amusa, 1993).

The VLF-EM profiles were conducted along North – South direction at the East and West side and along East – West direction at the North and South side outside the dump site with each profile consisting of three traverses at an interval of 15 m away from the dump site and each traverse covered the length of each side of the dump site except for traverse 8 (west side) which terminated at 100 m due to inaccessibility. At distance 800m away from the dump site, VLF-EM profile was also conducted with two traverses crossing each other which served as control profile.

Vertical electrical sounding (VES) was conducted at some points on the VLF-EM profiles after interpretation had been done on the VLF data. VES was conducted on each of the traverses at a point that showed red hot spot region (conductive zone) on the Karous-Hjelt imaging. This was done at the South and West side of the dump site except for the North and the East side. On the North side, the first and second traverses could not be sounded due to the topography of the ground but VES was conducted on the third traverse and extra two

VES were conducted at an interval of 15m away from it and from the dump site. Also, due to the inaccessibility at the East side, three VES were conducted at the North-East side at an interval of 15m away from the dump site. Two VES were also conducted on the control VLF-EM profile; one each on the two traverses (Fig 2). Water samples were collected from hand dug wells around the dumpsite, from well W (90 m East of the dumpsite), well X (45 m North of the dumpsite), well Y and well Z (35 m and 65 m South of the dumpsite respectively) and were analyzed to determine the concentration of heavy metals. The acquired VLF-EM field data were processed to simplify the obtained complex information with the aid of software program, Karous-HjeltFraser Filtering which suppressed the noise and enhanced the signal strength and also converted it to 2-D inversion image that showed subsurface structural fractures (conductivities). Apparent resistivity pseudo-sections have been prepared from the measured schlumberger sounding data, thus permitting a semi-quantitative idea of the resistivity distribution within the subsurface. Popoola et al (2011) described in detail the use of Schlumberger soundings in analyzing 2-D resistivity structures. Quantitative interpretation of the sounding curves involved partial curve matching and computer assisted inversion. The RMS misfit between the field and calculated data was generally less than 4%. Geoelectrical sections were also prepared from the layered model interpretation.

Results and Discussion

The representative results of the Karous-Hjelt filter 2-D inversion current density plots for traverses 10, 11 and 12 are presented in Figure 3, traverses 7, 8, and 9 are presented in Figure 4, traverses 4, 5 and 6 are presented in Figure 5, traverses 1, 2 and 3 are presented in Figure 6, traverses 13 and 14 are presented in Figure 7. The 2-D inversion shows the variation of apparent current density, and change in conductivity with depth. With such apparent current density cross-section plots, it is possible to qualitatively discriminate between conductive and


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Table 1: Summary of the Resistivity, Curve type, Number and Thicknesses of Soil Layers.

VES Point

Layer

Resistivity (? m) Thickness (m) Curve Type

1

1

513 12

234 5.4 H3

1487.7 -2

1

279.2 1.32

53.4 7.8 H

3

482.6 -3

1

141.8 1.3

2

6.1 2.6 H

3

1084.7 -4 1

164 0.82

594 4 A 3

1482.5 -5

1

147.6 0.8

2

228.7 2.4 A3

1205.1 -6 1

128 1

2

166.6 2.6 A 3

5646 -7

1

65.7 0.9 2

102.2 3.5

A 3

2043.4 -

8

1

110.8

1

2

79.9

3.8

H 3

1669.1 -

9 1 226.1 0.82 140 6.1 H3 1172 -

10 1 101.4 0.82 281.8 4.2 A3 709.1 -

11 1 99.5 0.8 2 120.9 3.3 A

3 944.1 -12 1 157.8 0.9

2 128.6 7.6 H3 1065.6 -

13 1 488 0.72 1414 2 3 456 6.4 A 4

1698 -

14

1

401.9 1.6 2

60.8 5.9 H

3

6561.8 -

resistive structures where a high positive value corresponds to conductive subsurface structure and low negative values are related to resistive one,

this is in agreement with Carpenter et. al, (1990).

Plotting was made on bilogarithmic sheets and partial curve matching method was used to obtain

initial data. These data were fed into WINRESIST 1.0 software for necessary iterations which synchronized and produced the respective thicknesses and number of layers, (Table 1).The quantitative interpretation of the sounding data gave three layers for all the VES except VES 13 which gave four geo-electric layers.

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Figure 3: Karous-Hjelt current density plots showing inferred fracture for the south of the dumpsite.

The results of the VLF-EM traverses showed decrease in conductivity with distance in a characteristic pattern from traverse 10 to traverse 12 (Figure 3), (Campbell et. al, 2006). At distance 85m to 115m in traverse 10 (Fig. 3a) showed a red hot spot (conductive subsurface structural trend) which also appeared faintly in traverse 11(Fig. 3b) but showed no trace in traverse 12 (Fig. 3c). A characteristic pattern of faint resistive subsurface structural layer appeared in traverse 10, 11 and 12 which was a deviation from the zero level of conductive subsurface layer of the soil along the traverse. There was a good correlation between

the results of VLF-EM and the DC resistivity method employed. The results of the vertical electrical sounding showed increase in resistivity for the second layer from VES 3 to VES 1. The results showed that VES 3, VES 2 and VES 1 have resistivity value of 6.1? m, 53.4? m and 234? m, respectively. The deep colours indicated higher conductivity or low resistivity. As we traverse from VES 3 through to VES 1, the values of resistivity increases which is an indication that the concentration of ions (heavy metals) decreased as we moved in this direction or away from the dumpsite (Campbell et. al, 2006).



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On the Karous-Hjelt imaging, Traverse 7 (Fig. 4a) shows a sharp red hot spot at a distance of 100m with a fading image (spread out) at 85m to 100m in-depth, on right side was a large resistive subsurface layer that is believed to have hindered the conductive zone from appearing in traverse 8 (Fig 4b). A faint conductive layer appeared in traverse 8 between distances 35m to 38m which was bounded on both sides by less resistivity layers. A moderately low conductive layer

Figure 4: Karous-Hjelt current density plots showing inferred fracture for the west of the dumpsite.

appeared in traverse 9 (Fig. 4c), this may suggest decreased in Pb ion concentration released at the dumpsite. The results showed that conductivity decreased across the traverses which complements the resistivity values for the first layer from VES 6 to VES 5 (Popoola et. al, 2011). VES 6 has a resistivity value of 128? m, 147.6? m for VES 5 and VES 4 showed an anomaly in its resistivity value of 164? m which suggests that the underlying layer was of lower conductivity.

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Figure 5: Karous-Hjelt current density plots showing inferred fracture for the north of the dumpsite.

Traverse 4 (Fig. 5a) showed a conspicuous red spot which indicates high level of conductive subsurface zone with a shadowy spread at distance 80m; it was bounded on both sides by non-conductive subsurface layer, distances 50m, 100m and 120m also showed outspread of conductive zone. Traverse 5 (Fig. 5b)on the other hand showed a point stain of conductive zone at distance 40m, distance 50m and 80m showed a less conductive layer compared to the zero level conductive layer of the soil along the traverse. Due to the inaccessibility, no vertical electrical sounding was conducted along traverse 4 and

traverse 5 but the result of traverse 6 (Fig. 5c) showed a good correlation with resistivity value of 65.7 ? m for the first layer of VES 7. Traverse 6 showed a conductive zone at distances 45m to 50m and at 80m to 85m between which was a vivid resistive subsurface layer with a shadowy outspread. The results of the vertical electrical sounding showed increase in resistivity for the second layer of VES 8 and VES 9 with value 79.9 ? m and 140 ? m, respectively. This was not unconnected with the fact that as we move away from the dump site, the concentration of ions decreases and so conductivity increases.



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Figure 6: Karous-Hjelt current density plots showing inferred fracture for the east of the dumpsite.

On the Karous-Hjelt imaging, traverse 1 (Fig. 6a) at distance 20m to about 40m showed a conductive subsurface zone, this was evidenced in the presence of the red hot spot (deep colour). Distance 50m and 110m shows a resistive subsurface layer with an out spread, a faint conductive zone can also be seen at distance 80m. Traverse 2 (Fig. 6b) showed a highly conspicuous red hot spot at distance 80m to 90m with a shadowy outspread from 60m to 105m in-depth. Conversely, a large resistive subsurface layer was seen at distance 120m to 140m. Traverse 3 (Fig. 6c) showed a faint resistivity layer at distance 20m to 40m and also at 80m to 90m but a conductive zone appear at 60m. The pronounced

resistive subsurface structures observed along and across the traverse are due to embedded rocks and outcrops seen on the ground (at the site) and these made it difficult to carry out VES on the traverses. The results of the VES along the North-east region (SIDE E) showed decrease in resistivity. Results showed that the resistivity value for the first layer of VES 10 was 101.4? m, 99.5? m for VES 11 and 157.8? m for VES 12 which was an anomaly. Also, the resistivity value for the second layer was 281.8? m and 120.9? m for VES 10 and VES 11, respectively but VES12 showed a resistivity value of 128.6? m. The bedding /topography of that layer might be responsible for this (Popoola et. al, 2011).

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Figure 7: Karous-Hjelt current density plots showing inferred fracture for the control survey.

On the Karous-Hjelt imaging, the results of the control profile showed that traverse 13 (Fig. 7a) at distance 65m to 80m, a conductive zone was observed but at distance 90m to 120m, a large resistive subsurface structural trend was seen. The result showed that the first layer of VES13 which is the topsoil had resistivity value of 488? m which is relatively high, the second layer was of higher resistivity value of 1414? m, the third layer had value of 456? m and the partially fractured layer i.e. the fourth layer has value of 1698? m. Traverse 14 (Fig. 7b) had a major conductive zone at distance 40m and a minor at 90m. At distance 100m to 125m, a less conductive zone was seen which suggested resistive geologic features, distance 50m to 60m are somehow more conductive, relatively; but VES 14 looks

aquiferous with resistivity value of 401.9? m and thickness 1.6m for layer 1, apparent resistivity value of 60.8? m and thickness 5.9m for layer 2 and apparent resistivity value of 6561? m for third layer which is the fractured bedrock.

The pictorial representation of Table 1is presented as a geo-electric section in Figure 8which showed the variation of the apparent resistivity of each layer with depth for all the VES conducted. It is the expected arrangement of the different types of soil as we move from the ground surface to the sub-surface.Also, the pseudo-section which is a more technical way of this representation is presented as contours in Figure 9 (i-v). The colour coded contours represent the changes in apparent resistivity with depth for each side of the dumpsite.



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Figure 9: Contour denoting changes in apparent resistivity with depth at; (i) side B, (ii) side C, (iii)sideD and (iii) side E. The contour lines represent the resistivity and the closer they are the more is the resistivity or conductivity is more when they are widely spaced apart as it is in Fig.9(i), side B.

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Figure 10: Variation of conductivity (due to the presence of contaminants) with distance away from the dumpsite from side: (i) side B, (ii) side C, (iii) side D and (iv) side E

In Fig. 10 (i), the apparent conductivity was relatively high and decreased with distance with an intercept I at 30 m. The intercept I suggests the distance at which conductivity was totally reduced. Fig. 10 (ii) on the other hand shows that the apparent conductivity bended away, that is, increased with distance. That the conductivity values were very low might suggest the direction of flow of underground water. Fig. 10 (iii) shows that the apparent conductivity decreases with increasing distance. Although it bended towards

the horizontal, it did not intercept it. This suggests that contamination was reducing with distance. Also further investigation might show the point of interception with distance. From Fig. 10 (iv),the apparent conductivity was nearly constant with increasing distance which then bends towards the horizontal axis distance. This suggests that the contaminant has travelled far along the horizontal axis but further investigation would show to what extent it travelled, this is in agreement with Jodeiri et. al,(2016).

Table 2: Comparison of the Water Analysis Results with Recommended Standards.

PARAMETER UNIT

LOCATION W.H.O (2004)

SON (2007) Well W Well X Well Y Well Z

pH 7.0 6.9 6.8 7.0 6.5 - 8.5 6.5 – 8.5 Pb mg/l ND ND 2.92 ND 0.01 0.01 Zn mg/l ND ND 0.483 ND 3.0 3.0 Cr mg/l ND ND ND ND 0.05 - Cu mg/l ND ND ND ND 2.0 1.0

ND: Not Detectable.



None of the heavy metals analysed was detected in wells X and Y. This was expected, based on the fact that there were outcrops at those sides. This in addition to the fact that wells were located towards the top of a slope and since underground topography was expected to follow the same pattern as that of the surface, one may expect that contaminants should flow away from these locations. Lead, Zinc, and Manganese were detected in well W; Lead had the highest concentration (2.92 mg/l) and Manganese the least (0.55 mg/l). Only Manganese of concentration 0.11 mg/l was detected in well Z. The concentrations of Zinc and Manganese detected in well W were above the Standard limit while the concentration of manganese detected in well Z was below the standard limit (Table 2). However, the pH of water in wells X, Y, W and Z were within the standard limit.

The VLF-EM results for the Northern region (side D) of the dumpsite showed high conductive subsurface fracture in traverse 4 and traverse 6 but traverse 5 showed a point stain. Also, VES 7 had its lowest resistivity of value 65.7? m, thickness 0.9m in the first layer, however, VES 8 and VES 9 had resistivity of value 79.9? m, thickness 3.8m and 140? m, thickness 6.1m respectively in the second layer. This shows a discontinuity of the spread of contaminant because conductive subsurface fracture should also be vivid in traverse 5 rather than a point stain. In the Eastern region (side A), VLF-EM results for traverse 1 showed a conductive subsurface fracture which was highly conspicuous in traverse 2 but appeared faintly in traverse 3. Although, VES were not conducted to give a complementary result but it is expected that the conductive subsurface fracture would be highly conspicuous in traverse 1 since it is nearer to the dumpsite. The electrical resistivity results for the North-east region (side E) showed decrease in resistivity for VES 10 and VES 11 meaning that conductivity increased at every 15m interval away from the dump site except VES 12 which showed an increase in resistivity. This was in opposite direction to the pattern of spread of contaminant since, VES 11 at 30 m from the dumpsite had higher conductivity

values for first and second layers than VES 10 which was conducted at 15 m away also VES 12 showed a low conductivity value. A trend of conductive subsurface zone was observed in traverse 7 and traverse 8 in the Western region (side C), but no trend was seen in traverse 9. However, the electrical resistivity results for the first layer showed that VES 4 had a resistivity value of 164? m which was relatively high compare with resistivity value 128? m and 147? m for VES 6 and VES 5 respectively. This is an anomaly which did not correlate with the VLF-EM results because the first layer resistivity value for VES 4 should lie between that of VES 6 and VES 5 since it was conducted 15 m farther than VES 6 and nearer to VES 5. In the Southern region (side B), the VLF-EM results showed conductive subsurface fracture in traverse 10 and traverse 11 but no trend in traverse 12, the electrical resistivity results also showed similar results. The result gives resistivity value 6.1? m, 53.4? m and 234? m and thickness 1.3m, 7.8m and 5.4m for VES 3, VES 2 and VES 1 respectively for the second layer. This shows that the conductivity in this region decreased with distance since the VES were conducted at an interval of 15 m outwardly.

Conclusion

The combination of Electrical Resistivity and VLF-EM methods was used successfully for the determination of heavy metal contamination outside the dumpsite. The results of the electrical resistivity model showed the vertical extent of the contaminated zones and subsurface contaminant pathways which indicated low resistivity (or high conductivity) values because of the presence of ions. The VLF which was very effective in the detection of conductors agreed with the results of the electrical resistivity imaging in detecting sub-vertical fractures as contaminant pathways (Jodeiri et al, 2016). The control experiment, VES 14, indicated that the second layer was a weathered and most conductive electrical layer among the layers in the weathering profile with resistivity 60.8? m and overburden thickness 5.9m. It could serve as aquifer for shallow groundwater exploitation. The results of the water

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analysis showed that well Ywhich was 35m away from the dumpsite (South region of the study area) was contaminated with Lead and Manganese, with a pH of 6.8. The contaminant in well Y flowed in the Southern region of the study area (side B).

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J.A. Adegoke,* O. O. Fasunwon and K. A. Oyekan Nigerian Journal of Science Vol. 49 (2015): 23-36ISSN 0029 0114

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